1. Field of the Invention
The present invention relates to a damper for continuously variably adjusting a damping force, and more particularly, to a damper for continuously variably adjusting a damping force for vibration occurring when driving on road.
2. Discussion of the Related Art
Generally, a suspension is equipped between a vehicle body and a vehicle wheel in order to improve ride comfort. The suspension includes a chassis spring that absorbs vibration and shock due to a road surface, and a damper that damps free vibration of the chassis spring in order to improve ride comfort.
The damper functions to absorb and quickly damp the vibration of the chassis spring.
Recently, an active control type suspension is widely used for a high-end vehicle. The active control type suspension adjusts a damping force by detecting a vehicle status using a sensor then getting feedback on the detected result thus electronically controlling a damping operation in response to the feedback.
There are various methods for electronically controlling a damping force of a damper. Generally, provided is a method of forming a fluid path in a piston valve traveling back and forth in a cylinder and adjusting a flow behavior of the fluid passing through the fluid path. As to this method, there is a method of adjusting a cross-sectional area of the fluid path, and a method of filling the cylinder with an Electro-Rheological fluid or a Magneto-Rheological (MR) fluid and adjusting a flow resistance of the fluid using an electric or magnetic property of the fluid.
FIG. 1 is a cross-sectional view illustrating a structure of a damper for continuously variably adjusting a damping force using an MR fluid according to the related art.
Referring to FIG. 1, the damper 10 includes a cylinder 12 that is open at one end thereof and has a hollow space along a length direction thereof, a piston rod 14 that is inserted into the cylinder 12 and moves back and forth, and a piston valve 16 that is coupled to a bottom portion of the piston rod 14 and moves back and forth in the cylinder 12.
A solenoid core 161 inducing an electromagnetic field is equipped in the piston value 16.
With respect to the piston valve 16, an inside of the cylinder 12 is divided into a compression space 12a below the piston value 16 and a tension space 12b over the piston valve 16. The compression space 12a and the tension space 12b are filled with an MR fluid F.
The MR fluid F is a fluid including metal particles having a size of about 3 micrometers to about 10 micrometers. The MR fluid F has a property that, when a magnetic field is produced around the MR fluid F, apparent viscosity of the MR fluid F changes due to the magnetic field.
Accordingly, by applying a magnetic field to the MR fluid F that passes through the piston valve 16 and flows into the compression space 12a and the tension space 12b thus changing the apparent viscosity, a damping force of the damper 10 is controlled.
In the related art damper 10, a volume compensation means is equipped in order that a volume change of the inner space of the cylinder 12 is compensated for according as the piston rod 14 moves back and forth.
The volume compensation means includes a floating plunger 18 that is below the compression space 12a and forms a buffer space 12c, and a buffer gas G filling the buffer space 12c. 
A sealing member 181 is equipped around a peripheral portion of the floating plunger 18 and contacts an inner circumferential surface of the cylinder 12 to prevent the buffer gas G from leaking. The floating plunger 18 is configured in the cylinder 12 such that the floating plunger 18 is capable of sliding up and down.
A nitrogen gas is usually used as the buffer gas G. The buffer gas G has a predetermined pressure to the extent that the floating plunger 18 is maintained at a reference height against a pressure applied through the piston valve 16. The reference height of the floating plunger 18 is a height when an external pressure is not applied.
In using the related art damper 10, shock and vibration due to a road surface when vehicle driving is transferred to the cylinder 12, thus conducted is a damping operation that a compression stoke in which the piston rod 14 and the piston valve 16 moves toward the compression space 12a and a tension stroke in which the piston rod 14 and the piston valve 16 moves toward the tension space 12b are continuously repeated.
In the compression stroke, the MR fluid F in the compression space 12a passes through the fluid path in the piston valve 16 and flows into the tension space 12b. In the tension stroke, the MR fluid F in the tension space 12b passes through the fluid path in the piston valve 16 and flows into the compression space 12a. Through these processes, vibration of the chassis spring is absorbed.
While the MR fluid F passes through the fluid path in the piston valve 16 and moves, the apparent viscosity of the MR fluid F changes due to the electromagnetic field produced by the solenoid coil 161 and flow property of the MR fluid F changes, thus a damping force of the damper 10 is controlled.
Since, in the compression stroke, a pressure is applied downwardly to the MR fluid F of the compression space 12a and the floating plunger 18 moves downwardly, and, in the tension stroke, a pressure applied to the MR fluid F of the compression space 12a decreases and the floating plunger 18 moves upwardly, compensation for volume and pressure in the cylinder 12 is made.
In other words, the buffer gas G is variably compressed in response to the pressure of the MR fluid F applied to the floating plunger 18 and gives the floating plunger 18 relative repelling force. Accordingly, the buffer gas functions like an air spring.
However, during the damping operation in the related art, the floating plunger 18 continues to move back and forth, and the buffer gas G is repeatedly compressed, thus energy is accumulated and temperature rises. Therefore, as the damping operation continues, the buffer gas G has a pressure more than an initial setting value, and further, a volume of the buffer gas G expands according to the pressure rise.
Accordingly, in the related art, due to the continuous damping operation, the temperature of the buffer gas G rises and the volume of the buffer gas G expands, thus the reference height of the floating plunger 18 moves upwardly. Therefore, the MR fluid F of the compression space 12a is pushed upwardly by the floating plunger 18. As a result, reference heights of the piston rod 14 and the piston valve 16 move upwardly.
Accordingly, a vehicle height becomes more than a normal vehicle height, and a vehicle's center of mass moves upwardly, thus running stability is degraded. Therefore, there is growing concern about vehicle overturn accident, and rolling action while running is intensified thus there is concern about degradation of ride comfort.